This invention relates to a method for the formation of electromigration resistant aluminum alloy conductors in semiconductor devices.
As is well known, electromigration in aluminum alloy conductors frequently is a cause of failure in high density integrated circuits. As a result of the electromigration, which occurs particularly under high DC current and high temperature conditions which occur very frequently in the operation of semiconductor devices, aluminum metal is transported by the current flowing through aluminum alloy containing conductors thereby causing the aluminum metal to form voids in certain areas and excess deposits in other areas. Because of this, the resistances of contacts in such devices are caused to increase to such an extent that there is an excessive amount of resistive heating resulting in the premature failure of the devices.
Various methods of increasing the electromigration resistance of aluminum alloy conductors have been attempted.
It has been found that the improvement in the lifetime was observed for the conductors consisting of a chain of (111) oriented Al single crystals. Such a "bamboo" structure has been obtained by for example by annealing semiconductive conductors consisting of a Al-0.5% Cu alloy at 450.degree. C. for 30 minutes. By use of this technique, described in Gat U.S. Pat. No. 4,331,485 increased lifetimes have been reported for conductors up to 4 .mu.m wide. Similarly, Pierce U.S. Pat. No. 4,352,239 shows a method of suppressing electromigration in aluminum conductors in semiconductor devices by heating the conductors to 480.degree. C. for about 50 minutes. However, these methods are considered to be too time consuming to be economically feasible.
In Howard et al U.S. Pat. No. 4,154,874 an increase in the resistance to electromigration was found when aluminum and a transition metal were co-deposited upon a supporting body at a very low pressure and in a substantially oxygen-free vacuum and the resultant body was then annealed at a temperature between about 200.degree. C. and 525.degree. C. for a time sufficient to form a region containing aluminum and transition metal compounds within the aluminum stripe. The specific minimum annealing time shown being 3 hours. This method has successfully been employed in suppressing electromigration but suffers from the defects of requiring a long annealing time and also requiring the presence of a transition metal such as titanium, zirconium, hafnium as well as other transition metals which tend to increase the cost of the device.
An additional method of reducing susceptibility of aluminum alloy semiconductor conductors to electromigration is that shown in Hall et al U.S. Pat. No. 3,848,330. According to the method of this patent aluminum alloy conductors containing a small percentage of copper are formed on a semiconductor device by evaporation techniques after which the device is heated to a temperature greater than 400.degree. C., particularly between 425.degree. to 475.degree. C., to alloy the copper into the aluminum and is then quickly cooled at a rate of at least 50.degree. C. per second to precipitate copper-rich A.sub.2 Cu precipitates out of the solution to form a fine grain structure having aluminum grains and grains of copper-rich precipitates interdispersed between the aluminum grains along the grain boundaries and triple points thereof. This approach in which there is no formation of large grains has not proved too successful in reducing electromigration nor can it be used for preventing electromigration in aluminum conductors containing no copper.